Sonic Blast (Game Gear)
This game has unused code.
Press Up, Up, Up, Down, Down, Down, 1, 2, 1, 2, Start at the title screen.
A build date is present at 0xC0:
GG SONIC 96 Ver E3 ROM 1996/05/01 @SEGA /Aspect Co.,Ltd
Oddly enough, the build date said it was a E3 Demo ROM, probably they didn't even bother to change it when finishing the game.
All of the 8-bit Sonic titles developed by Aspect contain a generic error handler routine at ROM 70. While the other games' routines consist of some variation on "print an error message and reset", the Sonic Blast handler simply freezes the game by going into an infinite loop. Since the game doesn't call this routine and the Z80 has no hardware error trapping, this code will only execute in the extremely unlikely event that a bug coincidentally causes the game to jump directly into it.
Present in the graphics for Sonic is this unused animation of the Strike Dash ability, which was in Sonic Chaos and Sonic Triple Trouble. All that remains of this move is this animation, which is stored with a vastly different object palette than used in the final game. (The above sprite animation was given a proper palette for visual clarity.)
The game's name was changed to G Sonic in Japan.